1. Field of the Invention
This invention relates generally to structures made of elemental carbon. It relates particularly to carbon nanotubes, and especially to a method for their production.
2. Description of the Related Art
Carbon nanotubes have drawn significant attention from the technological community with their unusual physical properties and wide range of actual and potential uses. Carbon nanotubes are made of a highly ordered sheet of carbon atoms rolled into a tube. This uniform structure gives the carbon nanotubes their unique properties. They have exceptional mechanical flexibility and strength, with reported tensile strengths of up to 100 GPa. These mechanical properties, along with their chemical stability, have allowed scientists and engineers to use carbon nanotubes as dopants in composites, and as tips for scanning force microscopy. Their nanoscale size and cylindrical shape have led to applications as a storage medium for gases and templates for nanowires. Their exceptional electron emission properties have led to successful use in field emission displays and luminescent tubes, while their electrical conductivity has encouraged their use as electrodes and in microcircuits.
The basic form of a carbon nanotube is a hexagonal network of carbon atoms. This network has been rolled to form a seamless cylinder with both end closed by a half of a fullerene molecule. A fullerene molecule is a closed hollow aromatic carbon compound that is made up of twelve pentagonal faces and differing numbers of hexagonal faces, which act as a cap on the end of the nanotube. Carbon nanotubes are grown in three forms: armchair, zigzag, and chiral.
There are two major groups of carbon nanotubes, viz., single-walled and multi-walled. A single-walled carbon nanotube has only one layer in the carbon lattice. Multi-walled carbon nanotubes are composed of concentric layers of single-walled carbon nanotubes.
When carbon nanotubes are produced, they are highly ordered and are well graphitized, i.e., their carbon lattice has very few imperfections, and they have properties very much like graphite.
Sumio Iijima first published his discovery of carbon nanotubes in 1991. One year before this, researchers in Heidelberg, Germany and Tucson, Ariz. reported a method for making large quantities of the carbon molecules called buckminsterfullerene or C60. This research justified the experiments Iijima had been conducting on the atomic-scale structure of carbon for over a decade. In 1991 Iijima experimented with the technique that had enabled the C60 researchers to make their new form of carbon. By passing electrical sparks between two closely spaced graphite rods, a process known as arc discharge, Iijima vaporized the rods and allowed the carbon to condense in a soot-like mass. When he looked at the mass through the microscope, Iijima found tiny tubes of pure carbon a few nanometers in diameter amongst the soot. These “nanotubes” were hollow, and had several layers. Roger Bacon had previously used the arc discharge method in the early 1960s to make “thick” carbon whiskers. Carbon nanotubes may be been formed by Bacon's experiments, but he lacked the high powered microscope required to see them. Iijima first saw multi-walled nanotubes; however, less than two years later he observed single-walled carbon nanotubes. The addition of a small amount of transition-metal power (cobalt, nickel or iron) favors the growth of single-walled nanotubes, a fact independently noticed by Donal Bethune and Iijima. A group at Rice University led by Richard Smalley completed the first mass production of carbon nanotubes. By 1993, multi-walled nanotubes had given way to single-walled versions, the properties of which are much easier to predict.
Since their discovery in 1991, carbon nanotubes have been manufactured almost exclusively by catalytic growth methods, which employ various metal catalysts to delay capping of the growing nanotubes, thereby allowing longer tubes to grow, which can be separated into single-walled tubes. Growth methods have included the arc-discharge method, whereby an electric current is created between a graphite anode and a graphite cathode, causing the graphite in the anode to vaporize and be deposited on the cathode in several forms, including carbon nanotubes. Chemical vapor deposition provides another significant growth method. For example, Jung et al. and Lee et al. have employed this method to grow carbon nanotubes by thermally decomposing acetylene in a furnace. As an example, a thin nickel layer is first deposited on an oxidized silicon wafer, and this substrate is then positioned in a horizontal flow reactor wherein it is pretreated by a flow of gases thereover. Acetylene is then added to the environment of gases, and the acetylene is decomposed to form nanotubes on the substrate.
Significant amounts of impurities are inevitable when carbon nanotubes are produced by the catalytic methods of the Related Art. In addition to increasing the cost of nanotube production, the employment of catalysts results in the presence of metal catalyst particles in the nanotube product, and additional purification processes are required for removal of these metal catalyst particles. For example, purification treatments employing acids such as nitric acid (Vaccarini et al.) or hydrofluoric acid (Colomer et al.) have been utilized for this purpose. Both Vaccarini and Colomer have reported, however, that their acid purification procedures have left some additional amorphous carbon from the pores of the metal catalyst particles, and this additional amorphous carbon requires an additional removal procedure.
It is accordingly a primary object of the present invention to obviate disadvantages presented by prior art processes and to provide a simple, inexpensive, energy-efficient method for the manufacture of high quality carbon nanotubes, which method does not employ a catalyst in the growth step of the procedure.